Xanthenes as fuel markers

ABSTRACT

A method for marking a petroleum hydrocarbon or a liquid biologically derived fuel; said method comprising adding to said petroleum hydrocarbon or liquid biologically derived fuel at least one compound that is a R1, R2, R3, R4, R5, R6, R7, R8, R9 and R10-substituted xanthene, wherein R1, R2, R3, R4, R5, R6, R7, R8, R9 and R10 independently are hydrogen, hydrocarbyl, hydrocarbyloxy, aryl or aryloxy; wherein each compound of formula (I) is present at a level from 0.01 ppm to 20 ppm.

This invention relates to a new method for marking liquid hydrocarbonsand other fuels and oils, as well as new compounds useful for thispurpose.

Marking of petroleum hydrocarbons and other fuels and oils with variouskinds of chemical markers is well known in the art. A variety ofcompounds have been used for this purpose, as well as numeroustechniques for detection of the markers, e.g., absorption spectroscopyand mass spectrometry. For example, U.S. Pat. No. 9,587,187 disclosesthe use of trityl aryl ethers for use in marking liquid hydrocarbons andother fuels and oils. However, there is always a need for additionalmarker compounds for these products. Combinations of markers can be usedas digital marking systems, with the ratios of amounts forming a codefor the marked product. Additional compounds useful as fuel andlubricant markers would be desirable to maximize the available codes.The problem addressed by this invention is to find additional markersuseful for marking liquid hydrocarbons and other fuels and oils.

STATEMENT OF INVENTION

The present invention provides a method for marking a petroleumhydrocarbon or a liquid biologically derived fuel; said methodcomprising adding to said petroleum hydrocarbon or liquid biologicallyderived fuel at least one compound of formula (I)

wherein R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R⁹ and R¹⁰ independently arehydrogen, hydrocarbyl, hydrocarbyloxy, aryl or aryloxy; wherein eachcompound having formula (I) is present at a level from 0.01 ppm to 20ppm.

The present invention further provides a compound of formula (II)

wherein R¹¹, R¹², R¹³, R¹⁴, R¹⁵, R¹⁶, R¹⁷, R¹⁸ independently arehydrogen, hydrocarbyl, hydrocarbyloxy, aryl or aryloxy; R¹⁹ and R²⁰independently are hydrocarbyl, hydrocarbyloxy, aryl or aryloxy; providedthat at least one of R¹¹, R¹², R¹³, R¹⁴, R¹⁵, R¹⁶, R¹⁷, R¹⁸, R¹⁹ and R²⁰contains at least seven carbon atoms; and that R¹¹, R¹², R¹³, R¹⁴, R¹⁵,R¹⁶, R¹⁷, R¹⁸, R¹⁹ and R²⁰ collectively contain fewer than 20 carbonatoms.

DETAILED DESCRIPTION

Percentages are weight percentages (wt %) and temperatures are in ° C.,unless specified otherwise. Experimental work is carried out at roomtemperature (20-25° C.), unless otherwise specified. Concentrationsexpressed in parts per million (“ppm”) are calculated on a weight/volumebasis (mg/L). The term “petroleum hydrocarbon” refers to products havinga predominantly hydrocarbon composition, although they may contain minoramounts of oxygen, nitrogen, sulfur or phosphorus; petroleumhydrocarbons include crude oils as well as products derived frompetroleum refining processes; they include, for example, crude oil,lubricating oil, hydraulic fluid, brake fluid, gasoline, diesel fuel,kerosene, jet fuel and heating oil. Marker compounds of this inventioncan be added to a petroleum hydrocarbon or a liquid biologically derivedfuel; examples of the latter are biodiesel fuel, ethanol, butanol, ethyltert-butyl ether or mixtures thereof. A substance is considered a liquidif it is in the liquid state at 20° C. A biodiesel fuel is abiologically derived fuel containing a mixture of fatty acid alkylesters, especially methyl esters. Biodiesel fuel typically is producedby transesterification of either virgin or recycled vegetable oils,although animal fats may also be used. An ethanol fuel is any fuelcontaining ethanol, in pure form, or mixed with petroleum hydrocarbons,e.g., “gasohol.” A “hydrocarbyl” group is a substituent derived from analiphatic hydrocarbon, which may be linear, branched or cyclic and whichmay have one or more hydroxyl or alkoxy substituents. Preferably,hydrocarbyl groups are unsubstituted. An “alkyl” group is a substitutedor unsubstituted saturated hydrocarbyl group having a linear, branchedor cyclic structure. Alkyl groups may have one or more aryl, hydroxyl oralkoxy substituents. Preferably, alkyl groups are unsubstituted.Preferably, alkyl groups are linear or branched, i.e., acyclic.Preferably, each alkyl substituent is not a mixture of different alkylgroups, i.e., it comprises at least 98% of one particular alkyl group.An “alkenyl” group is a substituted or unsubstituted hydrocarbyl grouphaving a linear, branched or cyclic arrangement and having at least onecarbon-carbon double bond. Preferably, alkenyl groups have no more thanthree carbon-carbon double bonds, preferably no more than two,preferably one. Alkenyl groups may have one or more hydroxyl or alkoxysubstituents. Preferably, alkenyl groups are unsubstituted. Preferably,alkyl and alkenyl groups are linear or branched, i.e., acyclic. An“aryl” group is a substituent derived from an aromatic hydrocarbon whichmay have aliphatic carbon atoms as well as aromatic carbon atoms. Anaryl group may be attached via an aromatic ring carbon atom or via analiphatic carbon atom. Aryl groups may have one or more hydroxyl oralkoxy substituents. A “hydrocarbyloxy,” “alkoxy,” “alkenyloxy” or“aryloxy” group is a substituent formed by adding an oxygen atom at thepoint of attachment of a hydrocarbyl, alkyl, alkenyl or aryl group,respectively (e.g., between an alkyl group and a dibenzofuran carbonatom). The number of carbon atoms in a substituent includes any carbonatoms which may be in alkyl or alkoxy substituents thereof. Preferably,the compounds of this invention contain elements in their naturallyoccurring isotopic proportions.

Preferably, R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R⁹ and R¹⁰ collectivelycontain at least one carbon atom, preferably at least two, preferably atleast three, preferably at least four. Preferably, R¹, R², R³, R⁴, R⁵,R⁶, R⁷, R⁸, R⁹ and R¹⁰ collectively have no more than 60 carbon atoms,preferably no more than 50, preferably no more than 40, preferably nomore than 35. Preferably, at least one of R¹, R², R³, R⁴, R⁵, R⁶, R⁷,R⁸, R⁹ and R¹⁰ has at least two carbon atoms, preferably at least three,preferably at least four, preferably at least five, preferably at leastsix. Preferably, hydrocarbyl groups are alkyl or alkenyl groups,preferably alkyl groups. Preferably, hydrocarbyloxy groups are alkoxy oralkenyloxy groups, preferably alkoxy groups.

Preferably, R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R⁹ and R¹⁰ independently arehydrogen, C₁-C₃₀ hydrocarbyl, C₁-C₃₀ hydrocarbyloxy, C₆-C₃₅ aryl orC₆-C₃₅ aryloxy; preferably hydrogen, C₁-C₃₀ alkyl, C₂-C₃₀ alkenyl,C₁-C₃₀ alkoxy, C₂-C₃₀ alkenyloxy, C₆-C₃₀ aryl or C₆-C₃₀ aryloxy;preferably hydrogen, C₁-C₃₀ alkyl, C₂-C₃₀ alkenyl, C₁-C₃₀ alkoxy, C₂-C₃₀alkenyloxy, C₆-C₂₀ aryl or C₆-C₂₀ aryloxy; preferably hydrogen, C₁-C₃₀alkyl or C₂-C₃₀ alkenyl; preferably hydrogen, C₁-C₂₅ alkyl or C₂-C₂₅alkenyl; preferably hydrogen or C₁-C₂₅ alkyl; preferably hydrogen orC₁-C₂₂ alkyl. Preferably, alkyl groups having at least 3 carbon atomsare linear or branched, i.e., not cyclic. In a preferred embodiment ofthe invention, at least one of R¹, R⁸, R⁹ and R¹⁰ is alkyl, and R², R³,R⁴, R⁵, R⁶ and R⁷ are hydrogen.

In using the compounds described herein as markers, preferably theminimum amount of each compound added to a liquid to be marked is atleast 0.01 ppm, preferably at least 0.02 ppm, preferably at least 0.05ppm, preferably at least 0.1 ppm, preferably at least 0.2 ppm.Preferably, the maximum amount of each marker is 50 ppm, preferably 20ppm, preferably 15 ppm, preferably 10 ppm, preferably 5 ppm, preferably2 ppm, preferably 1 ppm, preferably 0.5 ppm. Preferably, the maximumtotal amount of marker compounds is 100 ppm, preferably 70 ppm,preferably 50 ppm, preferably 30 ppm, preferably 20 ppm, preferably 15ppm, preferably 12 ppm, preferably 10 ppm, preferably 8 ppm, preferably6 ppm, preferably 4 ppm, preferably 3 ppm, preferably 2 ppm, preferably1 ppm, preferably 0.5 ppm. Preferably, a marker compound is notdetectible by visual means in the marked petroleum hydrocarbon or liquidbiologically derived fuel, i.e., it is not possible to determine byunaided visual observation of color or other characteristics that itcontains a marker compound. Preferably, a marker compound is one thatdoes not occur normally in the petroleum hydrocarbon or liquidbiologically derived fuel to which it is added, either as a constituentof the petroleum hydrocarbon or liquid biologically derived fuel itself,or as an additive used therein.

Preferably, the marker compounds have a log P value of at least 3, whereP is the 1-octanol/water partition coefficient. Preferably, the markercompounds have a log P of at least 4, preferably at least 5. Log Pvalues which have not been experimentally determined and reported in theliterature can be estimated using the method disclosed in Meylan, W. M &Howard, P. H., J. Pharm. Sci., vol. 84, pp. 83-92 (1995). Preferably thepetroleum hydrocarbon or liquid biologically derived fuel is a petroleumhydrocarbon, biodiesel fuel or ethanol fuel; preferably a petroleumhydrocarbon or biodiesel fuel; preferably a petroleum hydrocarbon;preferably crude oil, gasoline, diesel fuel, kerosene, jet fuel orheating oil; preferably gasoline. Preferably, the marker compound isadded as a solution in a solvent, preferably a hydrocarbon solvent.

Preferably, the marker compounds are detected by at least partiallyseparating them from constituents of the petroleum hydrocarbon or liquidbiologically derived fuel using a chromatographic technique, e.g., gaschromatography, liquid chromatography, thin-layer chromatography, paperchromatography, adsorption chromatography, affinity chromatography,capillary electrophoresis, ion exchange and molecular exclusionchromatography. Chromatography is followed by at least one of: (i) massspectral analysis, and (ii) FTIR. Identities of the marker compoundspreferably are determined by mass spectral analysis. Preferably, massspectral analysis is used to detect the marker compounds in thepetroleum hydrocarbon or liquid biologically derived fuel withoutperforming any separation. Alternatively, marker compounds may beconcentrated prior to analysis, e.g., by distilling some of the morevolatile components of a petroleum hydrocarbon or liquid biologicallyderived fuel.

Preferably, more than one marker compound is present. Use of multiplemarker compounds facilitates incorporation into the petroleumhydrocarbon or liquid biologically derived fuel of coded informationthat may be used to identify the origin and other characteristics of thepetroleum hydrocarbon or liquid biologically derived fuel. The codecomprises the identities and relative amounts, e.g., fixed integerratios, of the marker compounds. One, two, three or more markercompounds may be used to form the code. Marker compounds according tothis invention may be combined with markers of other types, e.g.,markers detected by absorption spectrometry, including those disclosedin U.S. Pat. No. 6,811,575; U.S. Pat. App. Pub. No. 2004/0250469 and EPApp. Pub. No. 1,479,749. Marker compounds are placed in the petroleumhydrocarbon or liquid biologically derived fuel directly, oralternatively, placed in an additives package containing othercompounds, e.g., antiwear additives for lubricants, detergents forgasoline, etc., and the additives package is added to the petroleumhydrocarbon or liquid biologically derived fuel.

In the compound of formula (II), R¹¹, R¹², R¹³, R¹⁴, R¹⁵, R¹⁶, R¹⁷, R¹⁸independently are hydrogen, C₁-C₃₀ hydrocarbyl, C₁-C₃₀ hydrocarbyloxy,C₆-C₃₅ aryl or C₆-C₃₅ aryloxy; preferably hydrogen, C₁-C₃₀ alkyl, C₂-C₃₀alkenyl, C₁-C₃₀ alkoxy, C₂-C₃₀ alkenyloxy, C₆-C₃₀ aryl or C₆-C₃₀aryloxy; preferably hydrogen, C₁-C₃₀ alkyl, C₂-C₃₀ alkenyl, C₁-C₃₀alkoxy, C₂-C₃₀ alkenyloxy, C₆-C₂₀ aryl or C₆-C₂₀ aryloxy; preferablyhydrogen, C₁-C₃₀ alkyl or C₂-C₃₀ alkenyl; preferably hydrogen, C₁-C₂₅alkyl or C₂-C₂₅ alkenyl; preferably hydrogen or C₁-C₂₅ alkyl; preferablyhydrogen or C₁-C₂₂ alkyl. Preferably, R¹⁹ and R²⁰ independently areC₁-C₃₀ hydrocarbyl, C₁-C₃₀ hydrocarbyloxy, C₆-C₃₅ aryl or C₆-C₃₅aryloxy; preferably C₁-C₃₀ alkyl, C₂-C₃₀ alkenyl, C₁-C₃₀ alkoxy, C₂-C₃₀alkenyloxy, C₆-C₃₀ aryl or C₆-C₃₀ aryloxy; preferably C₁-C₃₀ alkyl,C₂-C₃₀ alkenyl, C₁-C₃₀ alkoxy, C₂-C₃₀ alkenyloxy, C₆-C₂₀ aryl or C₆-C₂₀aryloxy; preferably C₁-C₃₀ alkyl or C₂-C₃₀ alkenyl; preferably C₁-C₂₅alkyl or C₂-C₂₅ alkenyl; preferably C₁-C₂₅ alkyl; preferably C₁-C₂₂alkyl. Preferably, alkyl groups having at least 3 carbon atoms arelinear or branched, i.e., not cyclic. Preferably, R¹², R¹³, R¹⁴, R¹⁵,R¹⁶, and R¹⁷ independently are hydrogen or C₁-C₄ alkyl, preferablyhydrogen. In a preferred embodiment of the invention, at least two ofR¹¹, R¹⁸, R¹⁹ and R²⁰ are alkyl, preferably at least three, and R¹²,R¹³, R¹⁴, R¹⁵, R¹⁶, and R¹⁷ are hydrogen. Preferably, at least one ofR¹¹, R¹², R¹³, R¹⁴, R¹⁵, R¹⁶, R¹⁷, R¹⁸, R¹⁹ and R²⁰ has at least eightcarbon atoms, preferably at least nine carbon atoms. Preferably, R¹¹,R¹², R¹³, R¹⁴, R¹⁵, R¹⁶, R¹⁷, R¹⁸, R¹⁹ and R²⁰ collectively containfewer than 19 carbon atoms, preferably fewer than 18, preferably fewerthan 17, preferably fewer than 16. In a preferred embodiment of theinvention both R¹⁹ and R²⁰ are C₁-C₂₂ alkyl; preferably both are C₇-C₂₂alkyl, preferably C₈-C₂₂ alkyl. In a preferred embodiment of theinvention, R¹⁹ and R²⁰ are C₁-C₄ alkyl and at least one of R¹¹ or R¹⁸ isC₇-C₁₆ alkyl, preferably C₈-C₁₆ alkyl.

The compounds of this invention may be prepared by methods known in theart, e.g., allowing xanthene or a substituted xanthene (e.g.,9,10-dimethylxanthene) to react with a strong base (e.g., alkyllithiums)to form a deprotonated precursor, then allowing said precursor to reactwith a compound having a carbon-halogen bond.

EXAMPLES Synthesis of Markers Synthesis of 9-ethyl-9H-xanthene (3) and9-hexyl-9H-xanthene (4)

A 150 mL Schlenk flask was charged with xanthene (2 g, 11 mmol) and THF(75 mL). The stirred solution was cooled in an ice-bath and n-buLi (1.6M in hexane, 7.5 mL) was slowly added over 2 minutes. The mixture wasstirred at 0° C. for 30 min and ethyl iodide (2.05 g, 13.17 mmol) wasadded. The reaction was allowed to stir for 1 h. The reaction wasquenched with H₂O (150 mL). The contents were placed in a separatoryfunnel and extracted with Et₂O (250 mL). The organic phase was isolated,dried (Na₂SO₄), filtered, and concentrated under reduced pressure.Purification was performed by column chromatography using hexane as theeluent. Mass of isolated product was 1.15 grams (49.8%). ¹H NMR (400MHz, Chloroform-d) δ 7.22-7.13 (m, 4H), 7.08 6.98 (m, 4H), 3.92 (t,J=5.7 Hz, 1H), 1.75 (qd, J=7.4, 5.7 Hz, 2H), 0.72 (t, J=7.4 Hz, 3H). ¹³CNMR (101 MHz, Chloroform-d) δ 152.47, 128.77, 128.73, 127.55, 125.28,123.14, 116.40, 116.34, 40.11, 33.60, 9.86.

Synthesis of 9-hexyl-9H-xanthene (4) was performed the same way but withhexyl bromide. Yield=1.60 grams (54.8%). ¹H NMR (400 MHz, Chloroform-d)δ 7.23-7.11 (m, 4H), 7.09-6.97 (m, 4H), 3.92 (t, J=6.1 Hz, 1H), 1.67(dt, J=8.3, 6.0 Hz, 2H), 1.38-1.04 (m, 8H), 0.79 (t, J=6.8 Hz, 3H). ¹³CNMR (101 MHz, Chloroform-d) δ 152.40, 128.72, 127.49, 125.89, 123.14,116.42, 40.96, 39.22, 31.85, 29.42, 25.60, 22.77, 14.19.

Synthesis of 9,9-dihexyl-9H-xanthene (5)

Compound 5 was synthesized as reported in the Journal of PolymerScience, Part A: Polymer Chemistry, 52(19), 2815-2821; 2014

Synthesis of 4-dodecyl-9,9-dimethyl-9H-xanthene (6)

A 150 mL Schlenk flask was charged with 9,9-dimethylxanthene (3 g, 14.26mmol), a stir bar, and THF (75 mL; anhydrous and degassed). The flaskwas cooled to −78° C. on a dry ice-acetone bath. To the flask, n-BuLi(9.8 mL of n-BuLi in hexane; 1.6 M) was added via syringe over 2 min.The flask was carefully removed from the dry ice-acetone bath andallowed to stir for 30 mins. The flask was then resubmerged in the dryice-acetone bath. To the flask, dodecyl iodide (4.42 g, 14.26 mmol) wasadded over 1 min via syringe. The reaction was allowed to stir and warmto room temperature overnight. The reaction was quenched with water (150mL) and the mixture was extracted with CH₂Cl₂ (250 mL). The organicfraction was dried (Na₂SO₄), filtered, and the solvent was removed on arotary evaporator. The product was purified by column chromatography ona 340 g silica Biotage column using a gradient of 0-5% CH₂Cl₂ in hexaneas the eluent. Mass of product as a colorless liquid=0.8 g (14.8%)

Laundering 5 and Analysis

A sample of 5 was made at 3 mg/l concentration in diesel that wastreated with basic alumina and filtered. The marked diesel sample wasmixed with laundering agent in the desired ratio. A magnetic stir barwas added to the sample and arranged on a multi position magnetic stirplate. All samples were stirred for 4 hours at 200 rpm. After four hoursall samples was let to settle for 30 minutes. An aliquot was taken fromtop and filtered through a 0.45 micron PTFE, filter. Laundered samplesalong with controls not exposed to laundering were analyzed by GC/MSwith the parameters below

Agilent 6890 Gas Chromatograph

Autosampler: Agilent 7683B Series

Detector: Agilent MSD 5973N mass spectroscopy detector

Column: DB-35MS, 15-m×0.25-mm ID, 0.25-μm film

Oven: Initial temperature 100° C.

Ramp 1 at 10° C./minute to 280° C., hold 0 min

Ramp 2 at 10° C./minute to 320° C., hold 5 min

Injection port: 280° C.

Transfer line: 280° C.

Injection Mode: Splitless

Carrier gas: Helium

Column flow rate: 1.4 mL/min, constant flow mode

Purge time: 20 min

Purge flow: 20 mL/min

Viscosity delay: 1

Injection volume: 1 μL

Acquisition mode: SIM

Solvent delay: 13 min

MS quad: 200° C.

MS source: 250° C.

Laundering Agent Comments 5 Al Oxide-Neutral 5% w/v 93.5 Al Oxide-Basic5% w/v 103.4 Bentonite 5% w/v 107.8 HCI Conc. 5% w/v 101.2 Fuller'sEarth 5% w/v 99.5 H₂O₂ −30% 50 v/v 96.1 Silica Gel 5% w/v 100.6 NaOCI(5%-bleach) 5% w/v 101.6 Activated carbon 5% w/v 95.8 Methanol 50% v/v106.3 Acetonitrile 50% v/v 103.9 H₂SO₄ Conc. 5% w/V 107.1 HNO₃ Conc. 5%w/v 106.8

Laundering and Analysis of Xanthene (1), 9,9-dimethyl-9H-xanthene (2),3, 4, and 6

Samples were made at 10 mg/l concentration in diesel that was treatedwith basic alumina and filtered. The marked diesel samples were mixedwith laundering agent in the specified ratio. A magnetic stir bar wasadded to the sample and arranged on a multi position magnetic stirplate. All samples were stirred for 4 hours at 200 rpm. After four hoursall samples were let to settle for 30 minutes. An aliquot was taken fromtop and filtered through syringe with 0.45 micron PTFE filter. Launderedsamples along with control which was not exposed to laundering wereanalyzed by GC/MS as described below

Quantitative analysis and separation of fuel markers from fuel matrixwas achieved with two-dimensional heartcutting gas chromatographymethod. A Deans Switch based on capillary flow technology is used toafford in-oven heartcutting capability. The method employs a DB-17HTcolumn (15 m×250 μm×0.15 μm) in the first dimension (D1) and a VF-WAXmscolumn (30 m×250 μm×1.0 μm) in the second dimension (D2). A flameionization detector (FID) was used as the detector in D1 and a massselective detector (MSD) was used for low level detection of markers theD2 column using single-ion monitoring (SIM) mode. The retention times ofthe markers in D1 which determine the heartcutting time was obtainedusing standards in xylenes. The full mass spectra of the individualmarkers were obtained to determine the most selective ion fragment thatgave the best sensitivity and selectivity for analysis of markers infuel matrix. Quantitation was performed using multi-point externalcalibration.

Laundering Agent Comments 1 2 3 4 6 Adsorbents Al Oxide-Neutral 5% w/v95% 100% 100%  97%  97% Al Oxide-Basic 5% w/v 99%  99% 102%  98% 101%Bentonite 5% w/v 96% 102% 105%  99%  92% Silica Gel 5% w/v 89%  94% 100% 97%  95% Fuller's Earth 5% w/v 99% 100% 100% 100%  87% Activated carbon5% w/v 90%  96%  48%  95% 100% Bases KOH (40%) 5% w/v 104%  103% 107%110%  93% NaOH (40%) 5% w/v 104%  103% 105% 105% 100% NaOMe (30% 5% w/v102%  102%  78%  98%  91% weight) in MeOH KOH and MPEG 5% w/v 92% 100% 50%  90%  91% 350 (0.5% weight) Acids H₂SO₄ Conc. 5% w/v 59%  94%  12%100%  95% HNO₃ Conc. 5% w/v  2%  95%  2%  2%  94% HCl Conc. 5% w/v 102% 102% 109% 103% 101% Oxidants H₂O₂ 30% 5% v/v 95% 101% 106% 102% 106%Bleach 5% v/v 90% 101%  61%  92% 107% (commercial) Solvents Methanol 50%v/v 106%  102% 108% 103% 103% Acetonitrile 50% v/v 97%  76%  97% 100%106% Temperature −30 C. 4 h and 96% 100% 107% 101% 108% filter 60 C. 4 hand 100%  101% 104% 100% 100% filter

The invention claimed is:
 1. A method for marking a petroleumhydrocarbon or a liquid biologically derived fuel; said methodcomprising adding to said petroleum hydrocarbon or liquid biologicallyderived fuel at least one compound of formula (I)

wherein R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R⁹ and R¹⁰ independently arehydrogen, hydrocarbyl, hydrocarbyloxy, aryl or aryloxy; wherein eachcompound of formula (I) is present at a level from 0.01 ppm to 20 ppm.2. The method of claim 1 in which R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R⁹ andR¹⁰ independently are hydrogen, C₁-C₃₀ hydrocarbyl, C₁-C₃₀hydrocarbyloxy, C₆-C₃₅ aryl or C₆-C₃₅ aryloxy.
 3. The method of claim 2in which R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R⁹ and R¹⁰ collectively have atleast one carbon atom.
 4. The method of claim 3 in which R¹, R², R³, R⁴,R⁵, R⁶, R⁷, R⁸, R⁹ and R¹⁰ independently are hydrogen, C₁-C₃₀ alkyl,C₂-C₃₀ alkenyl, C₁-C₃₀ alkoxy, C₂-C₃₀ alkenyloxy, C₆-C₂₀ aryl or C₆-C₂₀aryloxy.
 5. The method of claim 4 in which the compound of formula (I)is present at a level from 0.01 ppm to 10 ppm.
 6. The method of claim 5in which R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R⁹ and R¹⁰ independently arehydrogen or C₁-C₂₅ alkyl.
 7. The method of claim 6 in which at least oneof R¹, R⁸, R⁹ and R¹⁰ is alkyl, and R², R³, R⁴, R⁵, R⁶ and R⁷ arehydrogen.